Mass production of articles such as lenses for refracting electromagnetic waves, for example, in the optical or radio frequency bandwidths, is feasible by means of molding techniques. Plastics, especially polycarbonates known under brand names such as Lexan and Plexiglas, are especially useful as dielectric lens components in active antennas for receiving and amplifying radio waves.
The ability of lenses to refract electromagnetic waves is largely dependent upon the electrical characteristics, at the atomic level, of the material comprising the lens, and the shape of the lens. The shape of the lens must be controlled to a high degree of accuracy to ensure that the desired refracting effects are achieved. A lens the shape of which is distorted by even a relatively small amount will perform poorly and may not be effective or useful in the system of which they are a part.
Lenses tend to be homogeneous solid bodies having portions of varying thickness, and the difficulties of accurately molding a desired shape of a relatively thick body are well known. Molded parts tend to shrink as the material cools and solidifies within the mold, and the rate of shrinkage is proportional to the volume of material present. Thus thicker portions of the lens, having greater volume of material, will tend to shrink at a greater rate than the thinner portions of the lens, which have less volume. The effects of such differential shrinkage is manifested by sink marks, which are localized depressed areas which appear on the surface of the lens above the thicker regions of the lens where the rate of shrinkage is greater. The sink marks distort the shape of the lens and adversely affect the refraction performance, often rendering the lens totally useless. While adjustments to the mold tooling can be used to compensate for some degree of differential shrinkage, it is not generally possible to achieve acceptable results for relatively thick body lenses through mold tooling adjustment alone.
Furthermore, the thickness of the molded lens will have a direct effect on the cycle time needed to produce the lens. The thicker the lens, the longer it takes to cool and solidify, the time for cooling increasing in geometric proportion to the mass of material. Thus as lenses become thicker the cycle time rapidly increases. This results in lower production rates which increase the production costs for thick lenses significantly. It is thus also difficult to mold a relatively thick lens economically.
There is clearly a need for a method to economically mold relatively thick lenses which do not suffer significant shape distortion and thus will have adequate refraction performance.
It is an object of the invention to provide a method of molding parts.
It is another object of the invention to provide a mold useable in a method for molding parts.
It is again another object of the invention to provide a molded part.
It is also an object of the invention to provide a method of molding parts which do not have surface distortions due to shrinkage.
It is yet another object of the invention to provide a mold useable in a method for molding parts which do not have surface distortions due to shrinkage.
It is again another object of the invention to provide a molded part which does not have surface distortions due to shrinkage.
It is still another object of the invention to provide a molded part suitable for use as a lens for refracting electromagnetic waves.
It is yet another object of the invention to provide a method for molding a part having reduced cycle time.